336 Royal Society ; — Dr. Faraday’s Researches in Electricity. 
of the spectrum, and are less distinctly insulated ; two, perfectly 
round and well-insulated, at greater distances in the same direction; 
and one, very feeble and less satisfactorily made out, at no less a 
distance beyond the extreme red than 42*2 parts of a scale in which 
the whole extent of the Newtonian coloured spectrum occupies 539. 
March 19. — “ Researches in Electricity , Seventeenth Series ; on 
the source of power in the Voltaic Pile.” By Michael Faraday, Esq., 
D.C.L., F.R.S., &c. 
In this series, the author continues his experimental investigation 
of the origin of electric force in the voltaic pile. Having found 
abundant reason, in the experiments already described, to believe 
that the electricity of the pile has its origin in the chemical force of 
the acting bodies, he proceeds to examine how the circumstances 
which can affect the affinity of substances for each other, influence 
their power of producing electric currents. First, with relation to 
heat : — circuits were made of a single metal and a single fluid, and 
these were examined with a view to ascertain whether, by applying 
heat at one of the junctions, only thermo-currents can be produced. 
Some peculiar effects of heat are noticed and explained ; and several 
very necessary precautions in conducting these experiments are 
pointed out ; and it is found, when these are guarded against, that 
heat has a decided and distinct effect over the chemical affinities of 
the parts of a circuit subjected to its power, and a corresponding- 
influence on the electric current produced. This proceeds to such 
an extent, that, in some cases, either of two metals can be made po- 
sitive or negative with respect to the other in the same fluid, solely 
by virtue of this power of heat. 
The effect of dilution is then examined. For this purpose, only 
one metal and one fluid are used in a circuit ; but the fluid is 
rendered more dilute at one point of contact than at the other. 
First, it was ascertained that such dilution produces little or no ef- 
fect with metals which are not acted on by the electrolyte employed ; 
and the precautions requisite as to other points are then stated. 
But when these are observed, still dilution is found to have a most 
powerful influence on the results ; and, as the author believes, 
solely on account of its influence on the active chemical affinity. 
Thus copper in dilute nitric acid is positive with respect to copper 
in strong nitric acid ; and the same is the case with lead, silver, and 
other metals. It is not that the piece in the weakest acid is always 
positive with respect to that in the stronger acid ; for, in the first 
place, some very curious cases are given, in which a piece of metal 
ill acid of a certain strength is positive with respect to a piece of the 
same metal in acid, either stronger or weaker; and, in the next 
place, other cases are stated in which the piece in the medium acid 
is Jiegative with respect to the other piece in either stronger or 
weaker acid. The efect of dilution in nitric acid is such, that when 
certain different metals are compared together, one can, at pleasure 
l)e made positive or negative with respect to the other ; thus, of the 
five metals, silver, copper, iron, lead, and tin, any one of them can 
lie made either positive or negative with respect to any other ; with 
